Method and apparatus for X-ray production



Feb. 28, 1939. K, H, KINGDOM 2,149,093

METHOD AND APPARATUS FOR X-RAY PRODUCTION Filed June 9, 1937 2Sheets-Sheetl mg. l.

TEMPERATU/Tf DEG. C.

Inventor: Kenneth H. Kingdom,

b 3/ 5&9

y ig attorney.

Feb. 28, 1939. K. H. KINGDON METHOD AND APPARATUS FOR X-RAY PRODUCTIONFiled June 9, 1957 2 Sheets-Sheet\\\\\\m\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\zm\\\ 8 J a a lnventortKenneth l-LKin don, by I H15 Attorney.

Patented Feb. 2a, 1939 PATENT OFFICE METHOD AND APPARATUS FOR. X-RAYPRODUCTION Kenneth H. Kingdon, Schenectady, N. Y., assignor to GeneralElectric Company, a corporation oi.

New York Application June 9, 1937, Serial No. 147,287

3 Claims.

The present invention relates to improvements in methods and means forproducing X-rays and related radiations.

It is a. primary object hereof to make possible 5 the production ofX-rays at rates greatly in excess of those which have been realized inthe past.

In one aspect of my invention this is accomplished by utilizing as anX-ray source a discharge device having a cathode of a type which iscapable of generating almost unlimited quantitles of electron emission.One example of such a cathode comprises a body or pool of mercury.

It has been almost axiomatic to those familiar with the principles ofelectronics that discharge devices of the mercury pool type necessarilyoperate at too low a voltage drop to make X-ray generation possible.This is due to the fact that the use of a mercury pool (or equivalentcathode) inherently involves the presence in the discharge space of ionsof the cathode material, which ions tend to reduce the discharge voltagedrop to a value approximating the ionization potential of the material.(In the case of mercury this potential is about 10'volts.)

It is an important aspect of my present invention that a dischargedevice of the class in question may, under certain conditions and for atleast short intervals of time, be operated in such a manner as torealize potentials of X-ray-producing-magnitude in the discharge spaceand to provide X-ray-producing currents of extremely large magnitudes.Specifically, this is accomplished in a preferred embodiment of theinvention by the provision of high potential energy storage meanscapable of supplying extremely high-intensity current to a pool-typedischarge device during an initial discharge period.

The features of novelty which I desire to protect herein will be pointedout particularly in the appended claims. The invention itself, togetherwith further objects and advantages thereof, may best be understood byreference to the following description taken in connection with thedraw- 46 ings, in which Fig. 1 represents a discharge device suitablefor the practice of the invention in combination with an appropriatecircuit for energizing the same; Fig. 2 is a graphical representationuseful in explaining the invention; Figs. 3

50 and 4 illustrate alternative modes of applying the invention, andFig. 5 is a sectional view of an alternative form 01. discharge devicein connection with which the invention may be applied.

Referring particularly to Fig. 1, I have shown I lo a discharge devicecomprising an elongated glass envelope III which encloses a plurality ofcooperating discharge electrodes including an anode,

a cathode and a starting electrode. The anode, indicated at H, maysuitably comprise a plate or disk capable of X-ray emission uponbombardment by high velocity electrons. It is preferably constituted ofa refractory metal such, for example, as tungsten and may be supportedwithin the tube by a conducting member H, for example, of molybdenum. Inaccordance with the 10 present invention, the cathode indicated at I4 isof such nature as to be capable in its normal or steady-state operationof developing electrons at a sufficient rate to permit substantiallyunlimited energy to be transmitted through the dis- 1 charge device.Such a cathode is typically exemplified by a pool or otherself-reconstructing body of a readily vaporizable ionizable material,such as mercury.

It is characteristic of discharge devices of the U class described thatwhile they are capable of transmitting substantially unlimited energy,as previously specified, they inherently operate at low(non-X-ray-producing) potentials. Such operation is a result of the factthat in a steady- 25 state or non-transient condition sufllcient of thecathode material becomes vaporized'to permit the occurrence ofsubstantial ionization in the discharge space. Such ionizationneutralizes space charge and permits the passage of a discharge 30 atpotentials on the order of those which characterize an arc. Because ofthe apparent impossibility of developing high electron velocities withinsuch devices, it has heretofore been considered impracticable togenerate X-rays by 35 their use.

I have found that while the normal orsteadystate characteristics ofdischarge devices of the class under consideration are as stated in theforegoing paragraph, such devices may neverthe- 40 less exhibit duringtheir initial starting periods a brief transient state in which entirelydifferent conditions obtain. During such a starting period, highpotentials, sufiicient for X-ray production, may under proper conditionsbe established in the discharge space. The conditions necessary foraccomplishing this result are set forth in the following.

Referring again to Fig. l, I have illustrated apparatus which is adaptedto realize the method 50 of operation described in the foregoing. Thus,directly connected to the lead-in connections I! and ii of the anode andthe cathode, there is provided an energy storage means exemplified as acondenser ll,

During inoperative periods of I I the discharge device, this condensermay be charged to an X-ray producing potential by means of a highvoltage transformer l8 in combination with a rectifier i8 and acurrent-limiting resistor 20. It will be understood that the rectilleris may be of any known type, for example, a thermionic vacuum tube.

As a means for initiating a discharge through the discharge device thereis provided a starting electrode 22 suitably comprising a body ofsemiconducting material having a portion thereof in contact with thecathode during the normal operation of the device. If this electrode issub- Jected to a properly directed potential at a time when the anode IIis positive with respect to the cathode i 4 a discharge will be causedto take place between the anode and cathode. Particular means forimpressing such a potential are illustrated as comprising a condenser 23in combination with a battery 24 for charging the same and acurrent-limiting resistor 25. A manually operable switch 21 controllingthe electrode 22 makes it possible to initiate the discharge at desiredintervals.

For the desired mode of operation to be realized, it is necessary thatthe condenser I! be of relatively high capacity and capable of beingcharged to X-ray-producing potentials. (In a particular case I haveemployed with good success a condenser having a capacity of 0.015microfarad and charged to an operating potential of 100,000 volts.) Itis also necessary that the circuit connections 29 and 30, which connectthe condenser to the anode and cathode terminals of the dischargedevice, be of extremely low inductance and resistance in order that theyshall not constitute a limitation on the rate at which current may bedelivered to the discharge device. In connection with the dischargedevice itself, it is desirable to have the surface of the electrodesclean and free from contaminating impurities which might tend to modifythe breakdown characteristics of the device.

With these conditions fulfilled and with the condenser I1 charged asindicated, if the switch 21 is closed, a discharge will be caused totake place between the anode II and the cathode l4. If the energystorage means is capable of delivering energy to the discharge device atan adequate rate then X-ray emission will occur as a result of thebombardment of the surface of the anode l I. Due to the relatively greatelectron emission of a cathode such as that described (even during theinitial discharge period) the rate of X-ray production may be greatly inexcess of that heretofore realized with cathodes of other types. I haveactually obtained X-radiations at apparent instantaneous ratesequivalent to one million Roentgen units per minute, and much greaterrates are theoretically possible. This offers a remarkable contrast tothe radiation rates realizable with present commercial apparatus whichradiation is on the order of 50 Roentgen units per minute. It isanticipated that the great intensities of X-radiations which myinvention makes possible may be of great benefit in biological andtherapeutical applications.

In connection with the use of a mercury pool cathode for the purposesindicated, it has been found that the amount of X-ray radiationobtainable is a function of the temperature at which the mercury ismaintained. This is presumably due to the efiect which variations in thecathode temperature have on the residual concentration of mercury vaporsin the discharge space.

Presumably, with higher varcr concentrations the newly started dischargereaches its steadystate arc-like condition more rapidly than whensubstantially no mercury vapor is initially present.

The precise character of the variations obtained is illustrated in Fig.2, in which it appears that the amount of X-radiation obtainable with amercury pool cathode falls oi! rapidly at temperatures above about 0 C.For this reason, I prefer to operate at mercury temperatures below ,thisvalue, and in Fig. 1 I have shown an exemplary means for maintaining themercury temperature within the desired range. As shown, such meanscomprises a cooling coil 25 having an inlet 36 and an outlet 21 andarranged in heatexchanging relation with the cathode I4. At zero degreescentigrade the vapor pressure of mercury is about 0.16 micron.

In Fig. 3 I have illustrated an alternative means for utilizing thephenomena discussed in the foregoing. In the arrangement shown theenergy storage means is of the same character as that previouslydescribed and comprises a condenser 40 adapted to be connected to a highpotential source (not shown) through a current-limiting resistor 4|. Thedischarge device, however, differs from that previously described incomprising only an anode 43 and a cathode 44, the starting electrodebeing omitted. In series with the discharge device and between it andthe condenser 40 there is provided a spark gap comprising spaced spheres46 and 41, such spheres being separated by a distance which permitsbreakdown between them only upon application of a voltage of X-rayproducing magnitude. In order that the entire voltage of the energystorage source may be concentrated across the spark gap duringinoperative periods of the discharge device, the device is shunted by ahigh resistance 48 (say, 50,000 ohms).

In order to initiate a discharge between the anode 43 and the cathode 44with the arrangement illustrated two procedures are ossible. In thefirst, the spheres 46 and 41 may be maintained in fixed spatialrelationship and the voltage across the condenser 40 allowed to build upuntil breakdown of the spark gap occurs, whereupon substantially thefull voltage of the condenser will be impressed across the dischargedevice. In the second, the condenser 40 may be charged to a desiredvoltage and the spacing of the spheres decreased until a dischargeoccurs. In utilizing either of these methods X-radiation of thecharacter previously described may be obtained provided the rate ofcurrent supply to the discharge device during its initial dischargeperiod is sufliciently great.

In the still further modified arrangement shown in Fig. 4, I havecombined a spark gap as just described with discharge-initiating meansof the type illustrated in Fig. 1. In this arrangement, in which partscorresponding to those shown in Fig. 1 are similarly, numbered, the maindistinction consists in the fact that a spark gap 50 is included in thecircuit connections between the energy storage means and the dischargedevice. Under these conditions, before a discharge occurs and during theinoperative period of the discharge device a portion of the voltage willexist across the spark gap and another portion across the dischargespace between the anode II and the cathode I4. 7 As soon as the startingelectrode 22 is energized, however, the decrease in the drop across thedischarge space will raise the potential gradient across the spark gapand result in its immediate breakdown. This having occurred, the entirevoltage of the energy storage means will be impressed between the anodeII and the cathode l4, thus producing a steep wave front discharge. Itis an advantage of this arrangement that it permits using the ignitormethod of starting the discharge in a tube which would break down at amore or less random time if the full voltage of condenser II wereapplied to it without the series spark gap 50. Instead of the ignitor,any other of the customary means of initiating a mercury arc cathodespot may be used.

In Fig. 5 I have shown another form of discharge device which may besubstituted for that described in connection with Fig, 1. In thisembodiment there is provided an elongated glass envelope 55 which isclosed at its ends by means of metal members 56 and 51 sealed into theglass. The upper metal closure member acts as a support for an anode 58to which it is connected by means of a conducting structure includingshaft portions 60 and GI. These two shaft portions are connected bymeans of a conducting cylinder 62 which is of a diameter only slightlysmaller than that of the envelope itself and which provides a longnarrow space between the cylinder and the envelope whereby thepossibility of a discharge reaching the closure member 56 may beeffectively avoided. The lower closure member 5! acts as a receptaclefor a quantity of cathode material 64, suitably of mercury, and isprovided externally with cooling means in the form of a cooling coil 65as previously described.

In contact with the cathode material 64 there is provided a startingelectrode 66 comprising a tapered body of semi-conducting material. Inorder to avoid the danger of an unwanted discharge occurring between theelectrode 66 and the anode 58, there is provided a metal shield 68interposed between them. The shield 68 is preferably provided with arounded and polished surface whereby the danger of its acting as a pointof breakdown is reduced to a minimum.

It has been found that in operating an X-ray device of the typedescribed there is a tendency for tungsten or corresponding anodematerial sputtered from the anode surface to coat the envelope wall andto decrease the passage of X- rays therethrough. In order to avoid thisdifficulty, I have provided an X-ray-transmitting wall portion orwindow10 which is supported at the extremity of a'tubular extension IIproject-- ing from one side of the discharge envelope. The fact that thewindow 10 is relatively remote from the target surface necessarily tendsto prevent atoms of sputtered tungsten from reaching its inner surface.Furthermore'the mercury vapor which is ordinarily present within theextension H as a result of the partial vaporization of, the pool 64efiectively reduces the amount of tungsten which is able to find its wayto the outer end of the extension.

While I have in the foregoing referred primarily to mercury as asuitable cathode material, it should be pointed out that many othermaterials may be substituted therefor. For example, I have used tinsatisfactorily for this purpose, and one may employ alternativelygallium.

cadmium, or other similar metals. cathodes comprising these materialsare generically referred to herein and in the following claims as"pool-type cathodes", by which term I intend to designate a pool orother substantial body of the materials in question. In the use of allsuch materials for the purposes stated there should be present a sourceof a readily ionizable gas. This may comprise either a vaporizedcomponent of the cathode material itself, or a residual gas such asargon provided in the discharge envelope.

While I have shown particular embodiments of my invention, it will beunderstood that many modifications may be made by those skilled in theart without departing from the invention, and I aim by the appendedclaims to cover all such modifications as fall within the true spiritand scope of the foregoing disclosure.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. In combination, a discharge device comprising an anode constituted ofa material which is capable of effective X-ray emission upon bombardmentby high potential electrons and a pooltype cathode serving as a sourceof ionizable vapor, a condenser, circuit means for connecting thecondenser to the anode and cathode terminals of the discharge device,said circuit means having a substantially negligible impedance duringdischarge periods of the device, and means effective during inoperativeperiods of the device to charge such condenser to an X-ray producingpotential.

2. In combination, a discharge device comprising an anode constituted ofa material which is capable of effective X-ray emission upon bombardmentby high velocity electrons and a pool type cathode cooperativelyassociated with the anode, said cathode being capable in its normal orsteady state operation of developing electrons at a suflicient rate topermit substantially unlimited current to be transmitted between theanode and cathode at non-X-ray-producing potentials, a condenser,circuit means for connecting the condenser to the anode and cathodeterminals of the discharge device, said circuit means having asubstantially negligible impedance during discharge periods of thedevice, means effective during inoperative periods of the device tocharge such condenser to an X-ray producing potential, and means forinitiating a discharge through the device at desired intervals.

3. In combination, a discharge device enclosing an anode and a mercurypool cathode, means for cooling said cathode sufliciently to maintainthe normal vapor pressure in the envelope below about 0.16 micron ofmercury, a condenser, circuit means for connecting the condenser to theanode and cathode terminals of the discharge device, said circuit meanshaving a substantially negligible impedance during discharge periods ofthe device, means effective during inoperative periods of the device tocharge such condenser to an X-ray producing potential, and means forinitiating a. discharge through the device at desired intervals.

KENNETH H. KINGDON.

